Preparation of lignin-based polymer hydrogels: Stepwise removal of heavy metal-organic pollutant

The synergistic removal of wastewater containing heavy metals and organic pollutants remains a major challenge. Although multifunctional materials can enable one-step treatment, their high cost limits practical application, making sequential treatment with single-functional materials a feasible strategy. In this study, two lignin-based hydrogels were developed for stepwise removal of heavy metals and tetracycline (TC). Iron-manganese-modified lignin hydrogel (IM-LH) was prepared by copolymerizing acrylamide with lignin, in situ loading iron and manganese ions, and subsequent sulfuration, enabling efficient adsorption of Cd(II) and As(III). The immobilized enzyme composite hydrogel (HRP@HPAAM-LA) is made by chemically attaching horseradish peroxidase (HRP) to the pores and surfaces of a lignin-based semi-interpenetrating network carrier (HPAAM-LA), which is built through free radical copolymerization, using an EDC/NHS-mediated amidation reaction. This material can both adsorb and concentrate TC while letting HRP carry out oxidative degradation. The hydrogels were characterized by SEM, FTIR, XRD, XPS, and TGA, and their continuous removal performance and mechanisms were investigated. Both hydrogels exhibited uniform porous networks and abundant active sites. IM-LH showed maximum adsorption capacities of 86.20 mg·g − 1 for Cd(II) and 60.01 mg·g − 1 for As(III), retaining removal rates of 65.46% and 51.44% after 15 cycles. HRP@HPAAM-LA achieved an HRP immobilization rate of 78.13% and a TC removal rate of 92.22%, retaining 61.82% after 13 cycles. This study successfully prepared two lignin-based hydrogels, achieving the synergistic removal of heavy metals and organic pollutants. It provides a new method for the hierarchical and synergistic treatment of complex environmental wastewater, while also promoting the high-value utilization of biomass resources in environmental remediation.